if my sound system's frequency response is adjusted so that the result is flat when measured with C weighting, does that mean the low frequencies will have equal loudness to my ears, relative to the mid frequencies ?if not, which weighting curve should i use: flat or A weighting, or... ?

i should point out that my SPL meter is, on the "Flat" setting, +- 1 dB from 20 Hz to 12.5 kHz.

Yes, within the range of error of your SPL meter. The low-frequency errors inherent in the Radio Shack meter are significant. There is a correction table available on the internet if your meter is a Rat Shack model.

As you're no doubt aware, there are very large variations in low-frequency information from one source to another, CDs, analog vinyl, Dolby Digital 5.1 broadcasts and the like. I'm always adjusting my subwoofer level according to the source, by ear, to compensate for the vagaries of the source mix.

Yes, within the range of error of your SPL meter. The low-frequency errors inherent in the Radio Shack meter are significant. There is a correction table available on the internet if your meter is a Rat Shack model.

As you're no doubt aware, there are very large variations in low-frequency information from one source to another, CDs, analog vinyl, Dolby Digital 5.1 broadcasts and the like. I'm always adjusting my subwoofer level according to the source, by ear, to compensate for the vagaries of the source mix.

Alan

Yeah, I believe Ian mentioned in one of his videos re: multiple subwoofer set-up that the meter doesn't read properly below 50HZ anyway and is that possibly the case with any Room Eq System? I have always understand as well that similar limitation applies to the human ear as that was the purpose of the older receiver's "loudness control" to boost bass response to compensate, especially at low volumes so, in the end, you have to adjust the sub to personal taste.

Absolutely. The human ear is very insensitive to low bass at low volume levels. It always amuses me to see some enthusiasts searching out speakers that "deliver good bass at low volumes". Unless it has a built-in bass hump in the 80 Hz - 150 Hz range, no speaker delivers "good bass at low listening levels". I've always recommended Axiom's M3 bookshelf speaker to enthusiasts who want good bass at low listening levels because of its built-in modest bass hump.

If you want good bass at low volume, crank up the bass boost or increase the subwoofer level.

And yes, to be polite, Ian and I agree that Audyssey and similar auto-measurement-EQ systems are "error-prone" in the low-frequency octaves. Our suggestion has always been to listen to the speakers first after careful placement before engaging any Audyssey "correction" or auto-EQ.

I would imagine not everyone agrees with my approach, however, I have a Velodyne SMS-1 with the built-in parametric EQ that allows me to manually boost bass to some degree below that 50HZ level all the way down to 20HZ, BUT, one must make sure that the sub is capable of handling that extra thump. For the most part, from my calibrations in my particular set-up, a proper calibrated flat frequency response masks much of the 40HZ and below output requiring something to compensate for that loss and to be able to hear that "real deep" bass when it is part of the soundtrack.

Jacques, to the extent that your meter is accurate, "Flat" will be correct at low frequencies with regard to what the SPL level actually is. Since human hearing becomes progressively less sensitive at low frequencies(except at very high sound levels)the "C" weighting at typical levels more accurately duplicates the ear's sensitivity. For example, the C weighting is 3dB down at 31.5Hz, and a reading there using that weighting should be(intentionally, not an "error")3dB lower than a flat reading. "Correcting" this by adding 3dB isn't correcting an inaccuracy, but simply offsetting the effect of the built-in weighting, leading to a subjectively more nearly equal loudness.

These factors vary with frequency and sound level at each instant and a major benefit of Audyssey Dynamic EQ is to provide more sophisticated adjustments at each moment, with resulting significantly improved subjective bass response at very low frequencies.

exactly in accord with my experience.with a "flat" F. R. (as per Audyssey or my old Technics SH8000 sig. gen/SPL meter)I found out i was missing quite a lot of very low frequency signals, mainly in movies and in some music.

i increased the sub output in steps of 0.5 dB until i felt it was good. then i checked with the ring drop in LOTR Extended Blu-ray, starting again from flat, and came to use the same vol. increase for the sub: +3 dB re. flat config.

so far, i've used this setting with a few bass heavy movies played at Reference Level, and have had no problem at all, most probably because my room is small (around 1300 cu. ft.) and the gain used in the sub is low, which gives it a lot of headroom.

This is what i was slowly realizing was happening re. the difference between "C" weighting and "Flat". the "C" weighting did not seem to be very realistic comparing SPL levels on the meter and what my ears were hearing, so for a few days now i've been watching movies with the meter set to "Flat", and it just seems "right" to me.